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LM3881: VFLAG spike questions

Alfred Logico
Expert 4560 points
Part Number: LM3881

Hi Team,

My customer is planning on using our LM3881 to power the enable pin of the TPS22954. A concern of my customer is that the TPS22954 may turn on due to the Vflag votlage ramping up as it's input voltage ramps as well as seen in the table below for the LM3881.

The Vflag will be connected to the EN pin of the TPS22954 which has the below threshold voltage with min of 0.65V which is close to the VFLAG voltage spike.

It looks like the flag voltage does rise to 0.65V from that graph. Could you confirm that this is indeed the case? Also, what is the period of time that the voltage will be at or above 0.65V?

Thanks! 

Best Regards,

Alfred

  • 0
    TI__Mastermind 24075 points

    Hi Alfred, 

    The voltage in datasheet figure 6 should be true. 

    According to figure 6, the FLAG pin is high impedance when VCC votlage is below 0.65V. 

    One work around solution is to add a resistor below FLAG pin and ground to make the voltage lower. For example, add a 200k ohm resistor between FLAG pin and ground, the FLAG voltage will be around 0.4V when VCC voltage is 0.65V. When VCC voltage rises up and FLAG pin is released internally, the FLAG pin voltage will be VCC*2/3.

     Thanks

    Qian